The present invention relates to a focus detection method for use in an apparatus having a light receiving element array arranged on a predetermined focal plane of an imaging optical system or a plane equivalent thereto and a pupil dividing means for projecting light fluxes transmitted through first and second regions arranged symmetrically with respect to a plane including a light axis of an exit pupil of said imaging optical system onto adjacent light receiving elements in said light receiving element array, including the steps of calculating a lateral shift amount between images formed on a first light receiving element group which receives the light flux transmitted through said first region and on a second light receiving element group which receives the light flux transmitted through said second region, and detecting a focus condition of the image formed by means of said imaging optical system.
Heretofore, there has been proposed various methods for detecting the focus condition. For example, as shown in FIG. 1, a light flux transmitted through an imaging lens 1 is divided by a pupil dividing means 2 such as a stripe mask, a plurality of micro lenses or micro prisms, and the thus divided light fluxes are made incident upon a light receiving element array 3. The pupil dividing means 2 is arranged in such a manner that the light fluxes transmitted through different regions of an exit pupil are alternately made incident upon successive light receiving elements which construct the light receiving element array 3. Therefore, the images respectively formed on odd numbered light receiving elements 3.sub.1 -A, 3.sub.2 -A, . . . 3.sub.n -A and even numbered light receiving elements 3.sub.1 -B, 3.sub.2 -B, . . . 3.sub.n -B are made identical with each other in an in-focus condition, but are laterally shifted from each other corresponding to a lateral shift direction in a de-focus condition. Then, the focus condition of the imaging lens 1 is detected in response to an output difference between the odd numbered light receiving element group and the even numbered light receiving element group.
However, in this case, if it is assumed that a distance between an image plane 5 on which a projected image of an object 4 is formed by means of the imaging lens 1 is L.sub.0 and the image height i.e. a distance between an image point 5-1 corresponding to an object point 4-1 and an optical axis 7 is x, as shown in FIG. 2, a main light flux 8 which is introduced from the object point 4-1 to the image point 5-1 is inclined with respect to the optical axis 7 by an angle .alpha.(x)=tan.sup.-1 (x/L.sub.0) corresponding to the image height. Therefore, light intensities of the image 4 projected onto the adjacent light receiving elements become different from each other corresponding to the image height, so that a light intensity unbalance occurs and thus it is not possible to perform the focus detection in a highly accurate manner. Hereinafter, such an effect is called the image height effect.
Now, this undesired effect of the image height will be explained in detail with reference to FIG. 3.
The focus detection device shown in FIG. 3 which utilizes the stripe mask as the pupil dividing means 2 is constructed in such a manner that a pitch between light transmitting portions of the stripe mask is 2P, a pitch between the light receiving elements of the light receiving element array 3 is P, and respective light transmitting portions (in FIG. 3, only the portions 2i, 2j are shown principally) correspond to light receiving element pairs respectively (in FIG. 3, only the light receiving element pairs 3.sub.i -A; 3.sub.i -B and 3.sub.j -A; 3.sub.j -B which correspond to the light transmitting portions 2.sub.i and 2.sub.j respectively are shown). Therefore, the adjacent odd numbered light receiving element and even numbered light receiving element receive respectively the light fluxes transmitted through the different regions of the exit pupil.
In this case, if it is assumed that center points of the light transmitting portions 2.sub.i and 2.sub.j are 2.sub.i -S and 2.sub.j -S, the light receiving element pair 3.sub.i -A; 3.sub.i -B arranged near the optical axis 7 receives equal amounts of light flux transmitted through the light transmitting portion 2.sub.i with respect to the center point 2.sub.i -S. Contrary to this, the light receiving element pairs 3.sub.j -A; 3.sub.j -B arranged far from the optical axis 7 receive different amounts of light flux transmitted through the light transmitting portion 2.sub.j with respect to the center point 2.sub.j -S. That is to say, if the position of the light receiving element pair become far from the optical axis 7 as shown in FIG. 3, one light receiving element 3.sub.j -B receives the light flux transmitted through more than half region of the exit pupil of the imaging lens 1 and the other light receiving element 3.sub.j -A receives the light flux transmitted through less than half region correspondingly. Therefore, when outputs of respective light receiving elements in the light receiving element array 3 are successively read out from one end to the other end for an object having uniform contrast, the output distribution is shown by FIG. 4. In FIG. 4, only the outputs A.sub.i, B.sub.i of the light receiving element pair 3.sub.i -A, 3.sub.i -B corresponding to the light transmitting portion whose center point is made identical become equal, but the outputs of the light receiving element pair corresponding to the light transmitting portion whose center point is far from the optical axis 7 become unbalanced and a polarity of a difference therebetween is inverted with respect to the optical axis 7.
Therefore, if an intensity distribution of the object on the image plane is varied like a sine curve as shown in FIG. 5A, envelopes 3A and 3B of the odd and even numbered light receiving element groups are laterally shifted as shown by a solid and a dotted line, respectively, in FIG. 5B in the de-focus condition when ignoring the image height effect, but when the image height effect is taken into account these envelopes 3A and 3B are modulated by the light amount unbalance caused by the image height effect as shown in FIG. 5C and thus the detection accuracy is greatly affected.
As to the method for eliminating the drawbacks mentioned above, there has been proposed the method in Japanese Laid-Open Publication No. 130,524/80 wherein the light fluxes transmitted through the different regions of the exit pupil are made incident upon respective light receiving elements in the light receiving element pair at a constant pupil dividing ratio by correcting an inclination of a main light flux of the imaging lens with respect to the image height position by means of a correction lens arranged between the imaging lens and the pupil dividing means. FIG. 6 is a schematic view for explaining the optical system wherein the correction lens is arranged in front of the pupil dividing means shown in FIG. 3 using the stripe mask, and portions in FIG. 6 similar to those shown in FIG. 3 are denoted by the same reference numerals used in FIG. 6. In this method, the light flux transmitted through the imaging lens 1 is converted into parallel light flux by means of the correction lens 9 arranged between the imaging lens 1 and the stripe mask 9, and the parallel light flux is made incident upon the stripe mask 2 so that the inclination of the main light flux caused by the image height is corrected. However, in this method, since it is necessary to use the correction lens for correcting the inclination of light flux upon the the pupil dividing means, the apparatus becomes expensive in cost and further needs a space for the correction lens. Moreover the number of adjusting points during manufacture increases.
Heretofore, the incident light unbalance caused by the image height effect has been explained, but such incident light unbalance is also caused by a relative positional shift in the light receiving element array direction between the pupil dividing means and the light receiving element. In a normal case, the center point 2.sub.i -S of the light transmitting portion 2.sub.i must be positioned just above a center point 3.sub.i -S between the light receiving elements 3.sub.i -A and 3.sub.i -B, but as shown in FIG. 7 if the stripe mask is laterally shifted with respect to the light receiving elements 3.sub.i -A and 3.sub.i -B, the incident light amount projected onto the odd numbered light receiving element group is increased and that projected onto the even numbered light receiving element group is decreased. The relative positional shift between the stripe mask 2 and the light receiving element array 3 is caused by various factors such as nominal errors in a dimension and a pitch of the light transmitting portion of the stripe mask 2, those of the light receiving elements in the light receiving element array 3, and a possible lateral shift between the stripe mask 2 and the light receiving element array 3. If such relative positional shift occurs and a uniform light is projected onto the light receiving element array through the imaging lens, an output distribution of the light receiving element array becomes, for instance, as shown in FIG. 8 wherein the output distributions of the odd numbered light receiving element group and the even numbered light receiving element group are respectively varied in a linear manner. Therefore, when the intensity distribution of the object on the image plane is varied like the sine curve as shown in FIG. 5A, the respective output envelopes 3A and 3B of the odd and the even numbered light receiving element groups are modulated as shown in FIG. 9 by the light intensity unbalance caused by the relative positional shift shown in FIG. 8, and thus the detection accuracy is greatly affected. Moreover, FIG. 9 shows the envelope in the de-focus condition. Such light intensity unbalance based on the relative positional shift between the stripe mask 2 and the light receiving element array 3 cannot be corrected even if the correction lens is arranged between the imaging lens 1 and the stripe mask 2 as shown in FIG. 6.
As mentioned above, in the focus detection method which utilizes the image lateral shift between the images formed respectively on the light receiving element groups which receive the light fluxes transmitted through the first and the second regions of the exit pupil of the imaging optical system, by means of the pupil dividing means arranged in the light path between the imaging optical system and the light receiving element array positioned on the predetermined focal plane or the plane equivalent thereto, the incident light amount projected onto the light receiving element groups becomes unbalanced corresponding to the image height effect and the relative positional shift between the pupil dividing means and the light receiving element, and thus it is not possible to detect the focus condition in a highly accurate manner.